The present invention relates to a process for producing multilayered microparticles for marking and later identifying substances or objects of any desired type, even animals and plants, comprising the following process steps:
several marking layers are successively applied each in the liquid state onto a web-shaped or sheet-shaped substrate, each layer being dried and/or hardened before the next layer is applied, until a stack of layers of the desired type and in the desired order of the marking layers is obtained,
the substrate is separated from the stack of layers, and
the stack of layers is crushed into the multilayered particles without affecting the integrity of the complete series of the marking layers.
A process of the kind mentioned is known from GB-PS 1 568 699. In this known process according to the state of the art, common coating technology is used for applying the individual marking layers in the liquid state wherein for a preferred thickness of each marking layer a thickness of between about 5 and 50 xcexcm is disclosed. Foils, preferably Polyester foils, are disclosed as a substrate for the marking layers during the production of the layer stack. A wire-wound rod is used for coating the substrate with the individual marking layers in the liquid state. This wire-wound rod comprises a surface in form of a round wire wound around the outer circumference of the rod whereby a surface of the wire-wound rod is attained which as seen in a section is formed by a sequence of convex semi-circles. When this wire-wound rod is rolled on a plane substrate, the marking layer is applied on the substance or a marking layer which has been applied before and dried or hardened, which process is attained by the remaining intermediate spaces between the convex semi-circles on the one side and the substrate on the other side. By the surface tension within the marking layer which is still liquid, the thickness thereof becomes continuous and gets a substantial uniform dimension. To attain this continuous dimension of the layer thickness a relatively large thickness of the layer has to be used as otherwise other forces would outweigh the forces of the surface tension such that an automatic yield of a uniform thickness of the layer would be no longer attainable. In practical experience the marking layers applied in the liquid state comprise a thickness of about 25 xcexcm; after drying a thickness of the layer will remain which may decrease down to about 5 xcexcm wherein this value would be only attainable under laboratory conditions; in practical experience up to now no microparticles with a thickness of the layer below 8 xcexcm have been found. With common coating processes used up to now for the production of microparticles no thinner layer thicknesses of the marking layers are to be produced.
A further process of the kind mentioned before is known from U.S. Pat. No. 4,390,452. The process disclosed therein in the coating technology thereof corresponds with the process of the document mentioned above; the thicknesses of the layers of the marking layers in the examples shown in the US-PS are not smaller then 15 xcexcm per individual marking layer.
In the German published application DE 26 07 014 C2, a further process for the production of multilayered microparticles for the purpose mentioned above is described. In this process microparticles are produced according to a double peeling process. At first individual coloured plastic foils are stacked onto each other in the desired sequence, wherein the thickness of each foil is between about 12 and 200 xcexcm. Discs with a center aperture are cut out of each foil stack with the discs stacked on a thorn. This stack is heated to melt the foil layers with each other and to deform the material to the form of a rod which is peeled to form a strip. Again, discs with a center aperture are cut from this peeled strip which are again stacked on a thorn. Also, this stack is heated to melt the discs to form a rod which again is peeled to form a strip. This strip produced in the last step is put into a solvent, which selectively dissolves the substrate material layers which earlier have been integrated in the strip. In this way the individual microparticles are produced. It is obvious that this process takes much effort and is complicated and therefore has only a limited economy.
The processes according to the state of the art explained further above are substantially more economical, however, also in this case the productivity at the production is restricted and there is room for improvement. Furthermore all known production processes have the disadvantage that they are restricted to relatively large values regarding the minimal thicknesses of the individual marking layers to be produced, and therefore also regarding the total thickness of the multilayered microparticles. Applications for such microparticles are restricted for that reason alone because the microparticles are too large when manufactured to prior art. A further disadvantage of the known microparticles is that the heat resistance thereof is restricted to about 300xc2x0 C. whereby also many fields of applications are excluded where the identified substances or objects are subject to high temperatures.
It is therefore the object of the present invention to provide processes of the kind mentioned before which avoid the cited disadvantages, and by means of which microparticles with a smaller thickness of layers of the individual marking layers, and thereby also with a smaller total thickness, may be produced with a high economy without restricting the number of coding possibilities. Additionally the possibility is to be provided to produce microparticles with an improved heat resistance.
The first solution of the above object is attained in that the individual marking layers are applied in a printing process. Suitable printing processes, e.g., are the letter printing process, the rotogravure printing process, the flatbed printing process or screen printing processes with screens or templates.
In an embodiment of the process according to the invention it is proposed that each marking layer is applied in the pasty state instead of the liquid state. Applying the marking layers in the pasty state is easily possible with the printing processes mentioned above, in particular with the screen printing process. Advantageously it is possible to use absolutely new materials for the marking layers which are not available in the liquid state.
A preferred further development in this respect provides that each marking layer is applied in the thixotrope state. Hereby in particular the handling and processing of substances is simplified which are used for the marking layers. In order to enable the production of microparticles in particular with a higher resistance and/or higher chemical durability, it is furthermore proposed that glass powder and/or enamel powder with added heat-resistant colouring bodies is used as a base substance for the marking layers, and this base substance prior to its application is transferred into the pasty state by adding transfer lacquer or printing oil. By means of a selected heat treatment, microparticles are to be produced from this base substance whose individual marking layers are homogenous and stable as such and which at the same time have a firm coherence in the stack of layers.
A further process according to the invention which is also particularly suitable for the production of microparticles with higher heat resistance and/or higher chemical durability, is characterized in that each marking layer is applied in the dry state in the form of a powder instead of the liquid state by a bronzing process. This alternate solution of the object mentioned above advantageously enables the use of dry powders for the production of marking layers whereby further materials for the production of microparticles may be used which may not be produced or processed in a liquid or pasty state.
In a further embodiment of the above described process, advantageously glass powder and/or enamel powder with added heat-resistant colouring bodies is used as the base substance for the marking layers. In this way, particularly heat resistant microparticles may be produced which are especially well suitable for the marking of explosives or igniters for explosives.
A further alternate solution for the objective is attained in that the individual marking layers are applied by a spray painting process.
A fourth solution of the object is attained in that the individual marking layers are applied by a roll coating process.